The Smithsonian reports that the Monoprix supermarket, on the corner of the Rue Réaumur and the Boulevard de Sébastopol, uncovered hundreds of human remains when construction crews were digging under its basement floor. As the New York Times noted, the remains — almost exclusively bones — are from a medieval hospital first built in the 13th century.

France’s National Institute for Preventive Archaeological Research (or INRAP) is in change of excavating the site. So far INRAP crews have discovered eight graves with multiple people buried in each one. Seven of the graves contain the remains of five to eight people while the eighth grave interred an astounding (if not ghastly) 175 people.

The hospital that dug these graves is thought to be the Hôpital de la Trinité, a medieval hospital built during the reign of King Philip II. Until its destruction in 1790, the hospital served as a medical center, a shelter for religious pilgrims and the poor, and a children’s vocational school.

INRAP holds that the graves were carefully organized and the departed respectfully buried. Men, women, and children are all represented in the graves. Isabelle Abadie, the leader of the excavation team, found that ironically the smaller graves were less organized than the mass grave, which she feels indicates that the smaller ones were dug in haste.

“It could be the plague, it could be a famine, it can be many things at this stage,” Abadie said.

The team believes that the people in the graves passed away around the same time. DNA testing and carbon dating methods are being used to determine a more accurate date of their deaths.

One possibility was immediately ruled out, however: that the people died due to an armed struggle or persecution.

“There are no traces of trauma,” Abadie said. “So these aren’t deaths linked to an act of violence or war.”

Amazingly, the site is the first medieval cemetery to be excavated in Paris since the 18th century when the city exhumed and moved the remains of its medieval dead into the Paris Catacombs. The Catacombs, one of Paris’s most visited tourist attractions, is 20 meters deep, roughly equivalent to a five-story building.